A cryogenic gas, such as nitrogen, is supplied to a user from a variety of sources. If the user is a large volume user the gas is generally supplied, in the case of nitrogen, from an air separation plant. In any case, the primary gas supply system generally has a backup system to supply product gas in the event that the primary gas supply system becomes inoperative. A typical backup gas supply system comprises a low pressure liquid storage tank, a centrifugal pump to pump the liquid to the desired use pressure, and a vaporizer to convert the liquid to product gas. Another typical system utilizes a pressure building coil to pressurize the tank and thereby eliminate the pump.
In recent years there has arisen an increased need to supply cryogenic gas at pressures exceeding the critical pressure of the gas. For example, nitrogen, which has a critical pressure of 493 pounds per square inch absolute (psia), is now being used at pressures exceeding its critical pressure for molding and curing purposes in the tire industry. At such supercritical pressures, the conventional system employing a centrifugal pump is not effective because of the requirement for multiple stages and high rotational speeds.
The requirements for a supercritical pressure backup gas supply necessitate a different approach particularly if there is a power outage and the system is required to maintain product flow for a specified period of time. One approach to providing a supply of this nature is to maintain a liquid storage tank at the required supercritical operating pressure at all times. However, there are two serious problems to this approach. Both problems relate to the characteristics of supercritical fluids. When the storage pressure in a cryogenic tank exceeds the critical pressure, there are no longer two distinct fluid states--liquid and gas--which have a well-defined interface. Supercritical fluids have only a single phase which generally can be considered gaseous since it is compressible.
The first of these problems deals with tank contents gauging. Conventional methods used to determine liquid storage tank contents rely on a differential pressure measurement taken between the top and the bottom of the tank. Generally, the liquid phase density is considerably greater than the gas phase density and its saturation pressure can be determined or estimated. These differential pressure measurements can be translated into contents charts to determine the product inventory with reasonable accuracy. With supercritical pressures, differential pressure measurements cannot be directly translated into known contents. The temperature profile inside the tank is generally not known and, hence, the fluid density cannot be known. This is particularly true for tanks that sit idle for many days absorbing environmental heat leak. This uncertainty in backup product inventory reduces the reliability of the delivery system.
The second problem also relates to the lack of two phases in the storage tank at supercritical pressures. The conventional method of self-contained pressure maintenance which utilizes a density difference between the tank liquid and an external gas return circuit as the driving force for fluid flow will be ineffective.
Several methods have been proposed in order to reliably supply gas to a use point at supercritical pressure. One such method combines low pressure liquid storage with a positive displacement pump. However there are three characteristics of such systems which detract from their reliability when used with cryogenic fluids.
First, at the moment that the backup system is needed for operation, the pump is at ambient temperature and requires a period of cooldown and priming before it can operate successfully and deliver product at high pressure. This time period can often be many minutes. If the pipeline to the use point has little gas ballast, the pressure will quickly fall. Second, operation of a pumping system during backup periods requires electrical energy. During power outages that may occur, this energy must be supplied from a backup generator. Furthermore, electrical switchgear and controls must be employed increasing both the capital and operating costs. Third, cryogenic pumps require close attention to their maintenance for successful operation particularly their priming needs since they are pumping fluids whose temperatures are close to their boiling points. The requisite careful, scheduled preventive maintenance program to ensure that the pump will be operational when needed, further increases the system operating costs.
A second proposed method for reliably supplying supercritical pressure gas combines low pressure liquid storage with a multi-stage turbine pump. However, the problems associated with this system are similar to those discussed above with reference to positive displacement pumps. While the response to bringing the system on-line may be faster than that of the positive displacement pump depending upon design choices because separate insulated sumps filled with liquid cyrogen could surround the turbine pumps leaving them always cold for quick starting product losses during idle periods would be considerably greater due to the venting of liquid boil-off from the insulated sumps.
Yet a third proposed system involves a system comprising a dedicated bank of high pressure gas receivers. Backup operation can begin immediately and can last as long as the pressure in the receiver bank remains higher than the user requires. Electrical energy is not required and maintenance needs are almost eliminated. Unfortunately, this approach is practical only for short time periods, thus limiting the usefulness of this system.
A conventional pressure building coil takes a considerable time to achieve the requisite pressure and may not be operable during a power loss.
Clearly, a reliable and efficient system for providing high pressure gas to a use point is necessary and desirable.
Accordingly it is an object of this invention to provide an improved method for supplying gas to a use point at a high delivery pressure.
It is another object of this invention to provide an improved method for supplying a cryogenic gas to a use point at a high delivery pressure.
It is a further object of this invention to provide an improved method for supplying gas to a use point at a supercritical pressure.
It is yet another object of this invention to provide a gas supply system for accomplishing the above-stated objects.